1. Field of the Invention
The present invention relates to an optical pickup apparatus to be used to record or play back information signals on an optical recording medium such as an optical disk.
2. Description of the Related Art
Optical recording media are used as recording media for storing video/audio information and computer data. One type of digital optical disk widely used as one form of optical recording medium is the compact disc (CD). The diameter of the CD is 12 cm or 8 cm, the thickness of the protective layer protecting the information surface is 1.2 mm, the numerical aperture (hereinafter abbreviated as NA) of the objective lens used for recording or playback is 0.45, and the wavelength of the light source is 780 nm. Another type of digital optical disk called the Digital Versatile Disc (DVD) has a higher recording density than the CD and achieves a large storage capacity. The diameter of the DVD is 12 cm, the thickness of the protective layer protecting the information surface is 0.6 mm, the NA of the objective lens used for recording or playback is 0.6, and the wavelength of the light source is 650 nm.
With advances in information technology, the amount of information handled tends to increase, and there has arisen a need for a digital optical disk that surpasses the CD and the DVD in recording density and storage capacity. An effective method to increase the recording density of a digital optical disk is to increase the NA of the objective lens, shorten the wavelength of the light source, and thereby reduce the diameter of the light spot focused through the objective lens.
However, if the NA of the objective lens is increased, spherical aberration increases as the fourth power of the NA. The spherical aberration also varies when the thickness of the protective layer of the digital optical disk deviates from the specified value. A large spherical aberration would render recording or playback impossible; this limits the tolerances to which the protective layer and the optical system can be fabricated. To address this, it is proposed to reduce the effect of aberration by reducing the thickness of the protective layer to 0.1 mm and thereby suppressing the aberration caused by the tilting of the digital optical disk relative to the objective lens, and to achieve recording or playback on a high recording density digital optical disk by using an objective lens with a large NA and a light source with a short wavelength.
Related art in which such a proposal is implemented is disclosed in Japanese Unexamined Patent Publication JP-A 2000-111402 (2000). In the related art, an optical element is provided between the light source and the objective lens, which optical element is moved in the direction of its optical axis to compensate for the spherical aberration caused due to manufacturing errors in the protective layer thickness of the digital optical disk, and recording or playback is done on the high recording density digital optical disk by using an objective lens with a large NA. In the related art, the optical element is moved to compensate for the spherical aberration that occurs when the protective layer thickness of the digital optical disk contains an error of up to about 10 μm relative to the specified value. With the related art, if a CD or a DVD is to be recorded or played back, the spherical aberration due to the protective layer thickness differing depending on the type of digital optical disk must be compensated for by moving the optical element in the direction of its optical axis.
The difference in protective layer thickness between the CD and the high recording density digital optical disk is 1100 μm, and the difference in protective layer thickness between the DVD and the high recording density digital optical disk is 500 μm. As a result, if the spherical aberration caused due to the difference in protective layer thickness is to be compensated for, the amount of movement of the optical element must be increased. If the distance over which the optical element is moved to compensate for the spherical aberration increases, there arise such problems as the tilting of the optical element, the deviation of the optical axis, and difficulty in maintaining positioning accuracy, and in addition, the time required for positioning becomes longer, impairing user convenience. More specifically, the related art concerns a technique specially designed to record or play back the high recording density digital optical disk with a protective layer thickness as small as 0.1 mm by using an objective lens with a large NA, and therefore has the problem that it cannot be used to record or play back CDs or DVDs that have been distributed and stored in large quantities up to this date.
The related art that solves the above problem is disclosed in Japanese Unexamined Patent Publication JP-A11-120587(1999). The related art comprises two optical systems, one having an objective lens with a large NA which is used when recording or playing back the high recording density digital optical disk whose protective layer thickness is 0.1 mm, and the other having an objective lens with a small NA which is used when recording or playing back a CD or a DVD. Having two optical systems, the related art has the problem that the number of components increases, increasing the size of the apparatus as well as the manufacturing cost. Furthermore, since the low NA objective lens for the CD and DVD and the high NA objective lens for the high recording density digital optical disk are mounted on the same actuator, the moving part of the actuator increases in weight. The problem of this is that a large driving force becomes necessary to drive the objective lenses, which not only makes high speed driving difficult but poses an obstacle in reducing the power consumption.